Remove support for escaped-colon syntax in favor of using ${} for formatting in the future

[tecosaur]

I'm thinking it could be worth reverting 5dd8b9e considering the annoyances with requiring : to be escaped laid out in #1, and that this change would be annoying to make in a backwards-compatible way past the initial release (currently ${} is a syntax error, so it can be introduced in the future as a non-breaking change).

I'd like to get other people's thoughts on this, but if there aren't any comments against the change within the next fortnight or so I'll just do it.

[jariji]

I'll just mention I think Swift's "foo \(bar) baz" would probably have been a better syntax than $ for Julia string escape. Since \ is already special syntax we don't lose anything, whereas $ is otherwise valid in a string.

[tecosaur]

Yea, I personally prefer \(interpolate) too but I think that ship has already sailed.